It is known for a hyraulic circuit such as a motor vehicle brake actuating circuit to include a modulation device interposed between the hydraulic actuator such as a brake master cylinder and a hydraulic receiver such as a wheel brake cylinder. One form of such a device comprises a first chamber which in use is connected to the actuator and a second chamber connected to the receiver which the actuator is intended to control. Between the first and second chambers are a first communication passage controlled by a first valve which may be referred to as an isolating valve which is normally open, and a second communication passage controlled by a second valve which may be referred to as a rising pressure regulating valve and which is under a pilot control by electromagnetic actuating means. The device also has a passage providing communication between the second chamber and an outlet or tank; this communication passage is controlled by a third valve which may be referred to as a falling pressure regulating valve and which is also under a pilot control by electromagnetic actuating means. Each of the regulating valves is fixed to a respective guide stem which is slidably disposed in a guide bore formed for this purpose in the device body.
By way of the isolating valve which is normally open, there is normally a direct fluid flow connection between the first and second chambers and thus, in the case of a brake actuating circuit, between the brake master cylinder and the wheel cylinder.
When, as the result of the force applied by the vehicle driver to the brake pedal, the pressure in the second chamber, which is that connected to the braking receiver or wheel cylinder, increases dangerously to the point of reaching a critical value above which there would be a danger of locking the wheel or wheels being braked, the electromagnetic actuating means which provide the pilot control for the regulating valves, being subject to a control by control means responsive to the rotary speed of the wheel or wheels, then comes into operation. As a result, on the one hand the isolating valve and the rising pressure regulating valve close to interrupt the communication between the first chamber and the second chamber, while the falling pressure regulating valve opens to communicate the second chamber to the tank. The result of this is a fall in the pressure in the wheel cylinder which is thereafter isolated from its master cylinder.
In a first phase in the fall in pressure, the fall is abrupt, while in a second phase the fall continues in a controlled and moderate manner, under the control of the falling pressure regulating valve.
When the critical conditions which were likely to result in wheel locking have been removed, the electromagnetic actuating means ceases to operate and, under the action of resilient return means, the falling pressure regulating valve becomes operative again to close communication between the second chamber and the tank. This therefore permits a progressive rise in pressure in the second chamber and therefore in the wheel cylinder, under the control of the rising pressure regulating valve.
When the driver of the vehicle ceases to apply a braking pressure to the brake pedal, the isolating valve returns to its open position, unless it has already returned to that position previously.
Thus, when there are critical operating conditions which are close to those corresponding to wheel locking, such a modulation device causes a drop in the pressure in the wheel cylinder of the wheel or wheels which were in danger of locking, and this drop in pressure occurs irrespective of the braking pressure applied by the driver to the brake pedal; the modulation device then permits the pressure in the wheel cylinder to rise in a controlled manner when the critical conditions have been removed. The device is at all times capable of returning instantly to its rest or inoperative position in which it has no modulating action, as soon as the driver ceases to apply pressure to the brake pedal or as soon as the conditions of grip between the wheel and the road surface have been restored to a satisfactory value.
Previously known modulation devices of this type suffer from some disadvantages, essentially because the falling pressure regulating valve is in most cases formed by a simple needle member which controls a passage directly connected to a tank discharge. The needle member forming the falling pressure regulating valve is disposed generally in axial alignment with the rising pressure regulating valve and a single actuating means is provided between the needle valve and the rising pressure regulating valve and acts successively and alternatively on one or other of the valves, according to the conditions on which operation thereof depends.
Besides the fact that such a needle member is in practice delicate in construction, it is subjected on its two faces to different pressure. Accordingly, in order for the electromagnetic actuating means, which provide the pilot control action for the needle valve, to be capable of suitably ensuring opening of the needle valve, it is necessary that such actuating means should be relatively powerful and therefore cumbersome and expensive.
In addition, the fact that a single actuating means is used between the two regulating valves for actuation thereof inevitably gives rise to a time delay between the moment at which the actuating means ceases to act on the falling pressure regulating valve, at the end of the presssure fall phase, and the moment at which the actuating means acts on the rising pressure regulating valve at the beginning of the pressure rise phase which follows the pressure fall phase.
Although this time delay is minimal, generally some milliseconds only, the time delay can be sufficient in some cases to interfere with satisfactory operation of the device. In addition, during the time delay, the wheel cylinder remains at a constant and minimum pressure.
Finally, in some previously known constructions, for the very purpose of its displacement from one valve to the other, the actuating means disposed between said valves requires that a pressure that is not zero should be maintained in the second chamber. Consequently, the pressure in the wheel cylinder to which the second chamber is connected can never be reduced to zero, whereas it would sometimes be desirable, in particular on a bad road surface, for example on frost or ice, that the pressure in the wheel cylinder should be reduced to zero.